The present invention relates to a process and an apparatus for testing substances for potential toxicity.
During the drug discovery process a variety of tests are performed to avoid investments in further testing or development of substances which are likely to not be able to fulfil regulatory requirements, e.g. such substances may turn out to be toxic or otherwise be not suitable for the development of a drug. One of these test in a very early phase of the drug discovery process is the so-called “Ames test”.
In essence, the Ames test is a test for determining if a substance is mutagenic and is based on the assumption that a substance that is mutagenic (for the bacteria used in the Ames-test) may also turn out to be carcinogenic.
Although, in fact, some substances that cause cancer do not give a positive Ames test (and vice versa), the simplicity and low cost of this test makes it highly recommendable in the process of screening substance for possible carcinogenicity.
The respective bacteria (strains of salmonella) used in the Ames test carry a defective (mutant) gene making them unable to synthesize the amino acid histidine from the ingredients in the culture medium, thus making them unable to grow on a culture medium lacking histidine. However, some types of mutations can be reversed (back mutation), with the gene regaining its function. These revertants are then able to grow on a culture medium lacking histidine.
Many substances are not mutagenic (or carcinogenic) themselves but become converted into mutagens (or carcinogens) as they are metabolized by the body. This is the reason why the Ames test also includes liver enzymes. In case there is bacterial growth on the culture medium lacking histidine without liver enzymes having been added then the substance itself is mutagenic. If no such bacterial growth occurs without liver enzymes having been added but occurs with liver enzymes having been added then the metabolite of this substance is mutagenic. In either case, the substance is not further considered in further testing or in development of a drug. As already mentioned, the Ames test is routinely used in the drug discovery process due to its simplicity, low cost and its high probability that a substance (or its metabolite) which is marked by the Ames test as being cancerogenic is in fact cancerogenic. In addition, in some countries having performed the Ames test is a regulatory requirement that must be complied with in order to later on get the allowance for a drug to enter into the market.
For the reasons mentioned above, the Ames test is a well-established test in the drug discovery process. However, since nowadays very large numbers of substances are synthesized automatically in only small amounts the tests have to be performed in a highly efficient manner and with these small amounts of the substances. Typically these substances are provided in various differently diluted concentrations in micro-well plates (e.g. in micro-well plates having 96 micro-wells). However, the testing of these substances—or at least some essential steps thereof—is still performed manually to a large extent.